The first molecular complexes of holmium and erbium in the +2 oxidation state have been generated by reducing Cp' 3Ln [Cp' = C 5H 4SiMe 3; Ln = Ho (1), Er (2)] with KC 8 in the presence of 18-crown-6 in Et 2O at -35 °C under argon. Purification and crystallization below -35 °C gave isomorphous [(18-crown-6)K][Cp' 3Ln] [Ln = Ho (3), Er (4)]. The three Cp' ring centroids define a trigonal-planar geometry around each metal ion that is not perturbed by the location of the potassium crown cation near one ring with K-C(Cp') distances of 3.053(8)-3.078(2) Å. The metrical parameters of the three rings are indistinguishable within the error limits. In contrast to Ln 2+ complexes of Eu, Yb, Sm, Tm, Dy, and Nd, 3 and 4 have average Ln-(Cp' ring centroid) distances only 0.029 and 0.021 Å longer than those of the Ln 3+ analogues 1 and 2, a result similar to that previously reported for the 4d 1 Y 2+ complex [(18-crown-6)K][Cp' 3Y] (5) and the 5d 1 La 2+ complex [K(18-crown-6)(Et 2O)][Cp″ 3La] [Cp″ = 1,3-(Me 3Si) 2C 5H 3]. Surprisingly, the UV-vis spectra of 3 and 4 are also very similar to that of 5 with two broad absorptions in the visible region, suggesting that 3-5 have similar electron configurations. Density functional theory calculations on the Ho 2+ and Er 2+ species yielded HOMOs that are largely 5dz 2 in character and supportive of 4f 105d 1 and 4f 115d 1 ground-state configurations, respectively.